This KB article has some suggestions which might be useful -
How to improve the performance of your company file
Also, just to confirm, are you using Reckon Accounts Premier OR Reckon Accounts Hosted?
When you open the file what reports do you have left open from the previous session?
From the view menu make sure you have open window list ticked so you see what reports are open.
As the file opens, it will be generating each report left open from the previous session. If you have a large report or a report on a cash basis it will take a long time to open the file, as it generates these reports.
Make sure all reports are closed before you exit the file and then reopen and see if it is improved.